Each connection the Web Gateway makes to a destination begins with a TCP threeway three way handshake . This handshake must occur before the Gateway can send the HTTP request and if it fails then it will result in the “Cannot Connect” message.So,there might be some issue on these sites,if you are facing for all sites than it might be some issue.

we had also faced this issue and followed time exceed process,but no change in status it suddenly resolve the issue and sites are working fine.